There are a number of different types of semiconductor-based imagers, including charge coupled devices (CCD's), photodiode arrays, charge injection devices (CID's), hybrid focal plane arrays, and complementary metal oxide semiconductor (CMOS) imagers. Current applications of solid-state imagers include cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detector systems, image stabilization systems, and other image acquisition and processing systems.
CMOS imagers are well known. CMOS images are discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994); and are also disclosed in U.S. Pat. Nos. 6,140,630, 6,204,524, 6,310,366, 6,326,652, 6,333,205, and 6,326,868; assigned to Micron Technology, Inc., the entire disclosures of which are incorporated herein by reference.
Semiconductor imaging devices include an array of pixels, which converts light energy received, through an optical lens, into electrical signals. Each pixel contains a photosensor for converting a respective portion of a received image into an electrical signal. The electrical signals produced by the array of photosensors are processed to render a digital image.
The amount of charge generated by the photosensor corresponds to the intensity of light impinging on the photosensor. Accordingly, it is important that all of the light directed to the photosensor impinges on the photosensor rather than being reflected or refracted toward another photosensor (known as optical crosstalk).
For example, optical crosstalk may exist between neighboring photosensors in a pixel array. In an ideal imager, a light enters only through the surface of the photosensor that directly receives the light stimulus. In reality, however, some light intended for one photosensor also impinges on another photosensor through the sides of the optical path existing between a lens and the photosensor.
Optical crosstalk can bring about undesirable results in the images produced by the imaging device. The undesirable results can become more pronounced as the density of pixels in the imager array increases, and as pixel size correspondingly decreases. The shrinking pixel sizes make it increasingly difficult to properly focus incoming light on the photosensor of each pixel without accompanying optical crosstalk.
Optical crosstalk can cause a blurring or reduction in contrast in images produced by the imaging device. Optical crosstalk also degrades the spatial resolution, reduces overall sensitivity, causes color mixing, and leads to image noise after color correction. As noted above, image degradation can become more pronounced as pixel and related device sizes are reduced. Furthermore, degradation caused by optical crosstalk is more conspicuous at longer wavelengths of light. Light having longer wavelengths penetrates more deeply into the silicon structure of a pixel, providing more opportunities for the light to be reflected or refracted away from its intended photosensor target.
FIG. 1 illustrates the problem of optical crosstalk in a conventional backside illuminated imager. A conventional backside illuminated imager includes an array of photosensors 220, for example, photodiodes, formed within a substrate 290, a passivation layer 260, a color filter array (CFA) 250 and an array of microlenses 240. Ideally, incoming light 295 should stay within a photosensor optical path 223 when traveling through a microlens 240 to a respective photosensor 220. However, light 295 can be reflected within the respective layers of the imager and at the junctions between these layers. Incoming light 295 can also enter the pixel at different angles, causing the light to be incident on a different photosensor.
Optical crosstalk particularly problematic when it occurs within the substrate itself. This can occur in situations where a substantial amount of light is passing through the substrate 290, for example, in a backside illuminated pixel array or imager. For example, once light has passed the CFA layer 250, even small amounts of crosstalk can distort an image because adjacent pixels rarely filter out the same color. That is, if one portion of the spectrum of the incoming light is especially intense, crosstalk below the CFA layer 250 will redirect filtered light to photosensors 220 designed to measure a different color. Transmission of light through the substrate also suffers from electrical interference which can distort the signal further.
Accordingly, there is a need and desire for an improved apparatus and method for reducing optical crosstalk and related electrical interference in imaging devices. There is also a need to more effectively and accurately increase overall pixel sensitivity and provide improved optical crosstalk immunity without adding complexity to the manufacturing process and/or increasing fabrication costs.